Highway Drainage Design

• Proper drainage is an important consideration in highway design.
• Highway drainage involves removing and controlling excess surface and subsurface water from the roadway or right of way.
• Drainage includes intercepting and diverting water from the road surface and subgrade.
• Approximately 25% of highway construction funds are allocated to erosion control and drainage features like culverts, bridges, channels, and ditches.

Inadequate Drainage Effects

• Can cause damage to highway structures
• Can cause a loss of capacity
• Can cause visibility problems due to spray and retro-reflectivity
• Can cause safety issues like reduced friction and hydroplaning

Sources of Water for Highway Engineers

• Surface water, which includes rain or snow
• Groundwater, referring to water flowing in underground streams

Surface Drainage

• Includes all methods for removing surface water from the highway pavement and right of way.
• Transverse slopes facilitate the removal of surface water in the shortest time possible from the transverse direction.
• Longitudinal slopes require a minimum gradient in the highway's longitudinal direction for adequate slope in longitudinal channels, especially in cut sections.
• Longitudinal channels (ditches) are constructed along highways to collect surface runoff.
• Curbs and gutters control drainage, alongside other functions.

Highway Drainage Structures

• Structures are built to carry traffic over natural waterways within the highway right-of-way.
• Major drainage structures have clear spans exceeding 6m and typically involve large bridges, although multiple-span culverts can be included.
• Minor drainage structures have clear spans of 6m or less, including small bridges and culverts.

Sediment and Erosion Control

• Continuous surface water flow over shoulders, side slopes, and unlined channels often leads to soil erosion.
• Intercepting drains at the top of cuts help prevent erosion by capturing and diverting water.
• Curbs and gutters protect unsurfaced shoulders on rural roads from erosion.
• Turf cover on unpaved shoulders, ditches, embankments, and cut slopes is an efficient, economical way of preventing erosion on slopes flatter than 3:1.
• Slope and channel linings offer effective preventive action when highways face extensive erosion.

Hydrologic Considerations

• The basic hydrologic cycle starts with precipitation falling on the ground as water, snow, or hail, and returning to the air as vapor.
• Highway engineers focus on three rainfall properties: intensity (rate of fall), duration (length of time), and frequency (recurrence rate).
• Drainage should be designed for very rare storms to minimize overflowing.
• Decision-making factors include the highway's importance, traffic volume, and population density.
• Drainage area is the land area contributing runoff to a point where channel capacity is determined.
• Runoff coefficient (C) is the runoff-to-rainfall ratio for a drainage area, dependent on ground cover, slope, storm duration, prior wetting, and ground slope.
• Time of concentration (Tc) is the time it takes for runoff to flow from the most hydraulically distant point of the watershed to a point of interest.
• Time of concentration is affected by the size and shape of the drainage area, surface type, slope, rainfall intensity, and channelization
• Water travels through watersheds as sheet flow, shallow concentrated flow, open channel flow, or a mix.
• Travel time is the ratio of flow length to average velocity.

Runoff Determination

• The amount of runoff for any combination of intensity and duration depends on the type of surface.
• You can use topographical maps to compute watershed areas.
• The rational method calculates runoff based on average storm intensity, drainage area size, and drainage area surface type.

Hydraulic Design of Highway Drainage Structures

• The goal is to provide a suitable structure size that economically and efficiently handles runoff, while avoiding erosion and sedimentation.
• Hydraulic design of a drainage ditch for a given storm entails determining the minimum cross-sectional area of the ditch that will accommodate flow and prevent overflowing.
• Manning's formula is the most commonly used calculation.
• Most appropriate channel gradient range to produce the required velocity is between 1% and 5%.
• Sedimentation is usually a problem when slopes are less than 1% for most linings.
• Excessive erosion of the lining will occur when slopes are higher than 5%.
• Determining a suitable channel cross-section can be achieved using Manning's formula to find the flow depth required for the estimated runoff, often assuming a rectangular channel shape.

Culvert Design

• Culvert design considers location, watershed hydrology, economy, and flow control type.
• The most appropriate location for a culvert is in the existing channel bed.
• Avoid abrupt stream changes at the culvert's inlet and outlet
• Design flow rate depends on the storm and acceptable return period (frequency).
• Culverts are designed for the peak flow rate of the design storm.
• Tailwater is defined as the water depth at the outlet above the inside bottom of the culvert (culvert invert).
• Inlet control is the ability of the channel to store large quantities of water upstream from the culvert and may affect the culvert capacity design.
• The performance of a culvert under inlet control is influenced by factors such as inlet area, shape, configuration, and headwater depth.
• Outlet control is when a culvert’s barrel cannot transport as much flow as the inlet opening can receive.
• Performance of culverts under outlet control is affected by tailwater depth, roughness, area, shape, slope, and length.
• End treatment protects the pipe and embankment and improves hydraulics through armoring or funneling and addresses culvert failure due to scouring or piping.

Subsurface Drainage

• Systems drain water from the pavement structure in various forms.
• Water permeates through cracks and joints in the pavement to the underlying strata.
• Subsurface drainage design should be an integral part of highway design to avoid detrimental effects on slope stability and pavement performance.

Highway Sub-Drainage Systems

• Subsurface drainage systems are classified into longitudinal, transverse, and horizontal drains, plus drainage blankets and well systems.
• Longitudinal drains consist of pipes laid in trenches within the pavement structure, parallel to the centerline and these are used to lower the water table or remove seeping water.
• Transverse drains are placed below the pavement, perpendicular to the centerline, or skewed in a herringbone pattern but, the pavement's unevenness occurs during frost heaves.
• Horizontal drains relieve pore pressure at the slopes of cuts and embankments, made of perforated pipes inserted into slopes.
• Drainage blankets consist of vertical wells, that penetrate the ground where groundwater flows, which reduce water table and pore pressure by pumping the water out..

Design of Subsurface Drainage

• Summarize available data, determine water quantity for sub-drainage design, and determine the drainage system required.
• Determine capacity and spacing for longitudinal and transverse drains, and select filter material.
• Evaluate the design for economic feasibility and long-term performance.

Filter Requirements

• These requirements call for a coarse material drainage layer that allows water flow from the fine-grained subgrade soil to the coarse drainage layer.